Multi-product fractionation of roasted peanuts using supercritical carbon dioxide (SCCO2): evaluation of process alternatives

Abstract

Peanuts are grown for their nutritional value – the oil is particularly high in monounsaturated fatty acids, and the pulp is high in protein. When roasted, peanuts have a typical aroma that is valued in confectionary. They also contain a range of wax compounds. Peanut oil extraction is a multistep process that includes solvent extraction or cold pressing, degumming, neutralization, winterization, and deodorization. Industrially, hexane is used as the solvent in solid liquid extraction. The process is thus associated with drawbacks such as residual organic solvent persistent in the product, along with an adverse environmental impact and safety issues. Cold pressing is also used, but it is associated with low yields. Consumer trends necessitate exploring sustainable processes with minimum health and environmental risks. Substituting hexane with a dense form of carbon dioxide, known as supercritical carbon dioxide (scCO2) yields a process with no risk of residual organic compounds, nor the associated adverse safety or environmental concerns, since CO2 is generally accepted as safe for use in food and medical products manufacture. CO2 has a rather poor solvent power, yet it can be recycled without the need for additional process such as distillation to separate the product from the solvent – since the gas spontaneously separates from the liquid upon depressurization. The cost of the solvent is therefore minimum. In addition, its solvent power is a strong function of its density. Hence, its solvent power is easily adjustable through manipulating pressure. As a result, when CO2 is used as a solvent, high values of selectivity can be achieved. When these advantages are applied to peanut processing, then a question worthy of research arises: can the high selectivity’s obtainable using scCO2 as a solvent enable the processing of peanuts by successive extraction of each of the product? Hence, a hypothesis can be postulated: The high selectivity’s obtainable using supercritical CO2 can enable the components of peanuts to be removed successively from the feed material at different conditions; thus, peanut oil, free fatty acids, peanut aroma, and the protein rich residual could be obtained successively, and separately, from a batch of peanuts. The overall aim of this work was to prove (or disprove) this hypothesis experimentally. Questions arise from this hypothesis: what are the main compounds making up each of the envisaged products? Are the solubilities of each product in CO2 sufficiently different as to enable an acceptable separation? If so, what are the conditions under which these different separations can occur?

Can this process compare economically with the above-mentioned current process? These research questions are used to formulate the corresponding objectives. The typical chemical compositions of each of these was obtainable from the literature. Literature sources were also used to obtain the experimental equilibrium data describing the solubility – or the phase behaviour of the major compounds that make up each of these fractions in scCO2. These were used to perform an initial prediction of the possibility of separation using scCO2. The data was also used to perform an initial prediction of the conditions under which this separation could possibly occur. Experimental tests were performed, at pilot plant scale to validate the predictions. Finally, any technically feasible separation was then analyzed for its possible economic viability based on the typical operational costs found in the literature. The results obtained can be summarized as follows: when predicting the feasibility of separation of binary systems, oleic acid and triolein at 40°C produces better chances of enrichment between the concentration ranges of 0.1-0.282. The same behaviour was noiced for linoleic acid in triolein, and assuming that there’s no interaction at concentration ranges 0.125 and 0.236, then the mixture can be separated completely due to the dissolution of linoleic acid in CO2. Tripalmitate and oleic acid, pressures from 310 bar and above indicate that only tripalmitate can be enriched in mixture while at low pressures of 71 bar to 290 bar only oleic acid is enriched. At pressures above 290 bar up to 500 bar only tripalmitate can be enriched while at low pressure below 200 bar only linoleic acid can be enriched. For the validation of separation effeciency, the highest yield obtained at 150 was 18.39% while at 250 bar the yield increased to 47.8% and finally at 350 bar the highest extracted yield was 75.99%. The curves at 150 and 250 bar show a linear behaviour meaning the increase in the slopes represent a rise in solubility of the oil in solvent. A high temperature of 60⁰C produced the highest extraction yield by increasing the mass of the carbon dioxide used. The best separation would be obtained at 40˚C and varying pressures for different classes of compounds: 150, 250 and 350 bar for aroma compounds, free fatty acids and triglycerides respectively. While there are laboratory bench scale studies on SCF extraction of food grade peanut oil from peanut kernels, there is no information which could be found in the survey of literature on the economic feasibility of such a process. This research would provide reference data and a baseline study on the extraction of triglycerides from the oil in order to determine the techno-economic feasibility of the process. This information will be useful in process scale up.